The present invention relates to transmission line transformers.
The term xe2x80x9cTransmission Line Transformerxe2x80x9d (xe2x80x9cTLTxe2x80x9d) has come to mean various things over the past century. Some believe of it as a fabrication of select lumped LC elements while others see it generally as a filter network and still other see it as variations of the type of wire-wound transformer. Baluns, ununs and Blumleins are typical incarnations of a TLT. Regardless of the means, (i.e. lumped elements, wire or bonafide transmission line) the aim is always the samexe2x80x94to match impedance.
Impedance matching is important to prevent signal reflections which, at high frequencies and/or fast rise times, can produce false logic and data signals.
Prior TLT""s have required ferrite cores for coupling inductance such as described in Mourant, U.S. Pat. No. 5,886,589, and Hansen et al U.S. Pat. No. 4,488,136, the disclosures of which are hereby incorporated by reference.
A drawback with these types of TLT""s is that the ferrite forms part of the signal transmission path and hence the transformers are not as efficient as they should be due to parasitic inductive and capacitance losses. In Van Dine, U.S. Pat. No. 3,673,529 there is disclosed an impedance matching transformer for transmission lines which also relies on inductance. Furthermore, where usual TLTs are used, the number of conductors and complexity of the windings has limited impedance ratios to small values, e.g. 1:1 to 4:1 Baluns, Ununs and Blumleins.
Transmission line transformers are thus known. A general class of transmission line transformer which claims higher ratios, known as a Guanella Transformer, has an impedance ratio equal to n2, where n is the number of lines. A 16:1 example is illustrated in FIG. 1. The winding complexity and length of transmission line required for such transformers limits their construction to at most a few lines and low ratio. It is for this reason that baluns, ununs and other low impedance ratio TLTs have been limited to ratios of 4:1 or less.
Prior transmission line transformers have suffered from several other drawbacks. These drawbacks include high losses and low efficiency, narrow bandwidth, low operating frequency, limited impedance ratio, limited scalability and modularity. They are not conducive to mixing different types of transmission lines, and they are not well-suited to production methods or miniaturization.
There is a need for a transformer which can overcome the drawbacks noted above including reduction of parasitic losses, thereby increasing efficiency, operating frequency, bandwidths and impedance ratios.
There is, therefore, set forth according to the present invention a Series Transmission Line Transformer (STLT) which can provide for higher impedance ratios and bandwidths, which is scalable, and which is of simpler design and construction.
Accordingly a series transmission line transformer is set forth having input and output connections with a desired impedance relationship, voltage, and bandwidth. A first set of at least two transmission lines of equal length are connected together. Preferably, no more than three lines are used. Preferably, the lines have the same impedance, and preferably, they are all connected together in parallel on one end and in series on the other end. As such, a signal sent into the parallel ends will sum together (step up) on the series end, while a signal sent into the series end will step down at the parallel end. Preferably the lines are oppositely coiled about a closed core of magnetic material. Preferably the material is some formulation of soft ferrite. A second set of at least two transmission lines with the same caveats as stated above is attached to the first set of transmission lines. The junction between the two sets must be impedance matched. Preferably, the second and first sets have the same step up or step down polarity. (Exotic designs might not follow this scheme.) Preferably, the second set of lines are oppositely coiled about the same core as the first set. Furthermore, it must have the same volts per turn (common to a line, not differential) and flux orientation determined by helicity and voltage polarity. When a core is not used or required in this fashion (for strictly high frequency, narrow bandwidth), preferably individual cores or beads are threaded on each series-connected transmission line. Subsequent sets are attached in a chain or xe2x80x9cSeriesxe2x80x9d of sets to obtain the desired impedance ratio, hence xe2x80x9cSeries Transmission Line Transformerxe2x80x9d. Taken as a whole, the STLT performs much better than a chain of separate individual TLTs.
It has been found that the foregoing construction of connected sets in series and coiled about a common magnetic core, can provide a wide range of impedance ratios, (e.g from 4:1 to 1024:1) with relatively simple construction. Bandwidth, efficiency, fidelity and impedance ratio are mutually enhanced while keeping the STLT relatively simple. Only set pairs (or trios) of lines are used and sets are connected in series rather than individual lines.